1. Field of the Invention
This invention relates to methods to measure inertia for ion propulsion driven spacecraft.
2. Background of the Invention
This patent application is a continuation-in-part of the U.S. patent application Ser. No. 10/455,158, filed on Jun. 5, 2003, which is pending.
Gravimetric measuring devices are well known in the art, although they are highly specialized and expensive systems. To make a measurement of a gravitational field, a small, well-known mass is typically employed. Because any mechanical support of the mass will also drastically effect or even obscure the relatively weak effects of a gravitational field, typical means of mechanical support of a mass are not useful for gravimetrics.
For example, a typical scale to measure the weight of an item employs a spring to support the item. The mass of the item is estimated by measuring the compression of the spring when the item is suspended or supported by the spring. Variations in gravitational fields from one place to another, however, are so minute that they are immeasurable with even the most sensitive springs as the variations of the spring's force due to temperature, mechanical vibration, etc., are many times larger than the force variations in gravitational fields.
To this end, magnetism, and more specifically repelling magnetic forces, have been employed to suspend small masses for gravimetric measurements. Magnetism can be divided into three types of magnetic behavior: diamagnetism, paramagnetism, and ferromagnetism.
Ferromagnetism is the type of magnetism most commonly employed in modern, daily life. It is the result of naturally aligned intrinsic spin axes of individual electrons of the atoms of the material. Lodestone, iron and magnetite are some of the common materials used to create “permanent magnets”, as they exhibit their strong, dipolar magnetic properties under all conditions and temperatures, with or without the presence of other types of fields.
Initially, one who is unacquainted with magnetic theories may suspect that suspension or levitation of a small mass could be achieved using an arrangement of “permanent magnets”, or ferromagnets. In 1842, however, Samuel Earnshaw proved his theorem that there is no stable configuration to levitate permanent magnets using static magnetic fields.
Some quasi-stable levitation arrangements have been achieved by spinning the levitated mass, in which gyroscopic moments offset the inherent instability of the forces otherwise exerted on the suspended mass. Unfortunately, the gyroscopic forces are also large enough to offset or obscure the effects of small forces, such as variations in gravitational field, on the suspended mass. Additionally, energy must be induced into the spinning mass to keep it spinning over time, which may also obscure gravimetric measurements.
Diamagnetism may be viewed as an atomic version of Lenz's Law which provides that an electric current resulting from an applied magnetic field will be in a direction which sets up an opposing magnetic field. For example, if a dipole rod magnet (31) is passed or moved v(t) through an electrically conductive ring (30), a current i(t) will be induced in the ring in a direction which sets up a magnetic field (32, 32′) which opposes the movement v(t) of the rod magnet, as shown in the related patent invention in FIG. 1. This reactive current and opposing magnetic field is created regardless of the polarity of the inducing magnet.
All known elements are believed to exhibit some degree of diamagnetism. Most elements, however, do not exhibit noticeable or measurable diamagnetic properties. Under cryogenic conditions, such as 77 K, superconductive properties of many elements allow for substantial diamagnetic properties.
Under such cryogenic conditions, Lenz's law can be applied to statically levitate a small magnetic mass above a strong diamagnetic material, taking advantage of the Meisner Effect in which movements of the levitated mass result in a reactive and opposing field to correct for the movements, thereby leaving it in a stable position suspended above the diamagnetic material without any means of mechanical support. For example, FIG. 2, as shown in related patent invention, a small magnetic mass (41), such as a Samarium Cobalt magnet, can be levitated a distance d above a superconductive diamagnetic material (42) such as a ceramic Yttrium compound, which is superconductive at temperatures such as 77 K.
Such a cryogenic, diamagnetic arrangement has been employed by some gravimetric measurement systems, as the force between the levitated mass and the diamagnetic base is highly stable and constant, thereby allowing any differences in displacement between the base and the levitated mass to be attributed to the tiny variations in gravitational field. A laser interferometer may be employed to accurately measure the position of the levitated mass.
This type of gravimetric arrangement, however, is highly dependent on maintaining cryogenic conditions, which implies a need for a considerable supply of coolant such as liquid Nitrogen. Additionally, this type of system is difficult and expensive to operate due to the cryogenesis. Therefore, there existed a need in the art for a system and method for gravimetric measurement which avoids the need for superconductive conditions, materials, and supplies. The related patent application addressed these problems, and disclosed a structure and method of measuring minute changes in gravitational field using a levitated diamagnetic mass which requires operates at room temperature.
Another problem of minute energy measurement arises with respect to navigation requirements for spacecraft. Several organizations, such as the National Aeronautics and Space Administration (“NASA”) are investigating alternate methods of rocket propulsion without use of chemicals. Traditional methods of rocket propulsion using chemical reactions are unsuitable for long distance travel as the amount of chemical required to be transported would be prohibitively large.
Traditionally, chemical rocket engines work by combining fuel with an oxidizer. This makes a gas that expands and rushes out the back of the engine, generating the engine's thrust. Chemical engines are “mass-limited” engines. This means that the amount of power a chemical engine develops depends on the amount of fuel and oxidizer the vehicle can carry. When the supply of propellant is exhausted, the vehicle is no longer able to accelerate or maneuver.
An alternative type of propulsion engine under investigation for space vehicle motivation is known as an ion engine. Instead of using a chemical reaction to generate thrust, gaseous Xenon is given an electrical charge, or ionized, and then the ionized Xenon molecules are accelerated to a speed of about 30 km/second using an electromagnetic acceleration mechanism. When the Xenon ions are emitted from the exhaust of the ion engine, they push the spacecraft in the opposite direction, thereby providing a source of thrust without using a chemical reaction.
This type of propulsion means is expected to be highly suitable for long distance space travel, as it only requires a source of electrical energy which is readily converted by solar cells from sun light. Although the force they generate is relatively small when compared to traditional chemical-based rocket engines, when applied over time, ion drives are capable of providing great speed to a vehicle in near frictionless environment of space. As force is directly proportional to acceleration, spacecraft driven with ion propulsion systems will not undergo great amounts of acceleration in short amounts of time. As such, ion-driven spacecraft will require significantly longer amounts of time under much smaller acceleration to reach high velocities of travel. Under appropriate circumstances, however, ion propulsion is expected to be able to drive a spacecraft up to 10 times as fast as traditional chemical propulsion systems, and to dramatically increase the possible duration of space missions.
Navigation of long-range spacecraft, such as explorer craft, is particularly challenging. Compasses are not useful for space navigation, as they depend upon a planetary magnetic field to operate correctly. Global Positioning System (“GPS”) navigation depends upon a vehicle traveling within range of 3 or more GPS satellites in geosynchronous orbit about the Earth, which also are not available for spacecraft traveling far from the Earth. Therefore, navigation of such long-range spacecraft is typically performed by making inertial measurements during time, integrating the expected change in position, and using that as an offset from a known starting point.
Inertia navigation systems (“INS”) have at the heart of them an Inertial Measurement Unit (“IMU”). In turn, IMU's employ accelerometers to measure acceleration, or changes in velocity, usually in three orthogonal axes. If the mass, initial velocity and initial position of a vehicle are known, and the acceleration of that vehicle is measured over time, a subsequent position of the vehicle can be determined.
Because the thrusts of traditional chemical-based rocket engines is large for a short period of time, IMU's employed in the INS of these vehicles can easily detect and measure acceleration. This is typically done in modern IMU's using accelerometers comprised of specially designed integrated circuits (“IC”). In an accelerator IC, a volume of silicon or other base substrate is etched away from a finger of conductor to create a mass, albeit a small mass, at the end of a cantilever. The amount of mass, and length of the cantilever, determines how much the cantilever will bend in response to the IC being accelerated in a single axis. Three such cantilevers are usually employed to measure acceleration in three axes. The bend of each cantilever is carefully monitored and measured over time, thereby allowing calculation of acceleration, and ultimately determination of a position relative to a starting point.
However, due to the much smaller forces and acceleration characteristics of ion drives, traditional IMU systems are not expected to be sensitive enough to effectively provide for inertial navigation of space vehicles using ion drives. Additionally, cryogenic conditions may exist at some positions in space, but extremely hot conditions can be expected to be encountered by space vehicles as they pass near energy sources such as stars.
For these reasons, there exists a need in the art for an accelerometer sensitive enough to measure extremely small amounts of acceleration, under extremely cold as well as extremely hot operating conditions, without the requirement of any fuel or power source other than electrical energy, such that inertial navigation for ion-driven spacecraft can be achieved.